X-ray tube having focusing cup with non-emitting coating

ABSTRACT

An x-ray tube is described including a focusing cup electrode coated with a high work function material, such as platinum or gold, to prevent the field emission of electrons from such cup. The method of applying the non-emitting coating is preferably sputtering or ion plating, but may also be electroplating followed by vacuum fusion in the case of gold or other low melting point metals.

United States Patent [191 Atlee et al.

[451 Jan. 1,1974

[ X-RAY TUBE HAVING FOCUSING CUP WITH NON-EMITTING COATING [75]Inventors: Zed J. Atlee, Oak Brook; Roy F.

Kasten, Jr., Elmhurst, both of 1]].

[73] Assignee: Picker Corporation, Cleveland,

Ohio

[22] Filed: Aug. 30, 1972 [21] App]. No.: 284,735

[52] U.S. Cl 313/57, 29/25.l4 [51] Int. Cl. HOlj 35/06 [58] Field ofSearch 313/57 Primary Examiner-Roy Lake Assistant ExaminerDarwin R.Hostetter AttorneyStephen W. Blore et al.

[5 7] ABSTRACT An x-ray tube is described including a focusing cupelectrode coated with a high work function material, such as platinum orgold, to prevent the field emission of electrons from such cup. Themethod of applying the non-emitting coating is preferably sputtering orion plating, but may also be electroplating followed by vacuum fusion inthe case of gold or other low melting point metals.

7 Claims, 3 Drawing Figures HIGH WORK FUNCTION [MATERIAL (E9, PLOYAU.)

PATENTEDJAH 1 m4 3783.323

HIGH WORK FUNCTION [MATERIAL (E PLovAu) 8 EXPOSURE PULSER :0 26 [5V8 l2x RAY TUBE X-RAY TUBE HAVING FOCUSING CUP WITH NON-EMITTING COATINGBACKGROUND OF THE INVENTION The subject matter of the present inventionrelates generally to x-ray tubes employing heated filament cathodescontained within a focusing cup electrode, and in particular to suchx-ray tubes in which a layer of high work function material is coated onthe focusing cup to prevent the field emission of electrons therefrom.

Thermionic x-ray tubes having rotating anodes are operated at extremelyhigh voltages, typically on the orderof kilovolts, so that there is atendency for the focusing cup to emit electrons by field emission to theanode or to the cathode filament when the cup is negatively biasedrelative to such filament. This is a particular problem in condenserdischarge x-ray systerns and other systems employing coaxial cables ortransformers of high secondary capacitance because the high voltage isstored in the capacitance across the x-ray tube so that any fieldemission from the focusing cup causes a high current discharge. Thefield emission of electrons from the focusing cup to the filamentcathode can destroy such cathode, particularly if it is a thoriatedtungsten filament cathode which is easily damaged by evaporation ofthorium from the filament or contamination of the filament by thedeposit of evaporated material from the focusing cup. In addition,during manufacture the unactivated filament cathode may be damaged byfield emission from the focusing cup during seasoning as described incopending U. S. patent application Ser. No. 228,951, filed Feb. 24,1972, by Z. J. Atlee et al.

These problems are avoided in the x-ray tube of the present invention byemploying a non-emitting coating of high work function materials, suchas platinum or gold, on the surface of the focusing cup electrodeincluding the inner surface portions within the cup which areimmediately adjacent to the cathode filament. In the preferredembodiment, a platinum coating is employed because of its higher workfunction and higher permissible operation temperature. The platinum isapplied to the focusing cup by sputtering or ion plating which avoidsmelting the underlying focusing cup metal which would happen if a fusioncoating method were employed due to the high melting point of platinum.Previously, it has been suggested tht the anodes of high voltagerectifier tubes can becoated with a thin layer of gold over a thickerintermediate nickel layer provided on such anodes by electroplating andsubsequent heating below 780 Centigrade, as discussed in U. S. Pat.3,611,523 of E. S. Den Dulk, patented Oct. 12, 1971. However, heatingabove this temperature causes a low melting temperature alloy of goldand nickel to form which no longer has a high work function. However,this is impractical for coating the focusing cup of an x-ray tubebecause frequently such focusing cup is processed and operated at highertemperatures.

It is, therefore, one object of the present invention to provide animproved xray tube of longer useful lifetime in which the focusing cupelectrode is coated with a non-emissive layer of high work functionmaterial.

Another object of the invention is to provide such an x-ray tube inwhich the coating of non-emissive material is applied to the focusingcup by a method which maintains the high work function of the material,re-

sults in good adherence and provides a smooth surface on such focusingcup.

Still another object of the present invention is to provide such amethod in which the low emissive material is applied to the focusing cupbase: material by sputtermg.

A further object of the invention is to provide such a method using ionplating.

A still further object of the invention is to provide such a method inwhich a low emissive material is applied to the focusing cup byelectroplating followed by fusion.

An additional object of the present invention is to provide the focusingcup electrode of an x-ray tube with such non-emissive coating of a highmelting point material, such as platinum, without melting the underlyingbase material.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention will be apparent from the following detailed description of preferred embodiments thereof and from the attacheddrawings of which:

FIG. 1 is a plan view of an x-ray tube having a focusing electrode madein accordance with the present invention, with parts broken away forclarity;

FIG. 2 is a partial horizontal section view taken along the line 22 ofFIG. 1 on an enlarged scale; and

FIG. 3 is a schematic diagram of the electrical circuit of a capacitordischarge x-ray apparatus employing the tube of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS As shown in FIG. 1, one embodimentof an x-ray tube made in accordance with the present invention includesan evacuated envelope 10 of glass containing a rotary anode 12 and athermionic cathode assembly 14 supported at the opposite ends of suchenvelope in a conventional manner. Thus, the rotary anode 12 is attachedby a rod 16 to a bearing sleeve 18 of magnetic material which isrotationally mounted on an inner shaft 20 for rotation by field coils(not shown) external to the envelope. The bearing sleeve18 and anode 12are supported on anode support shaft 20 which extends through aglass-to-metal seal in the left end of the envelope 10 for applyingpositive voltage to such anode.

The cathode assembly 14 includes a filament cathode 22 in the form of acoil of tungsten, thoriated tungsten, or other suitable electronemissive material. The filament cathode 22 is supported within a notch24 in the focusing cup electrode 26. The focusing cup electrode 26 isprovided with a cup-shaped focusing aperture 28 flaring outwardly fromthe notch 24. As a result, the electrons emitted by the filament 22 arefocused by cup 28 onto a target surface 30 of the rotary anode 12 tocause x-rays to be emitted therefrom and transmitted through the side ofthe envelope 10.

The focusing cup electrode 26 of the present invention includes a basemember 32 of steel, nickel, molybdenum, or other suitable refractorymaterial including ceramic, such as alumina, having a non-emissivecoating 34 of high work function material, such asplatinum or gold. Thenon-emissive coating 34 is provided on the upper surface of the focusingcup electrode 26, including the inner surface of the focusing cupaperture 28 and notch 24. The purpose of this non-emissive coating 34 isto prevent the field emission of electrons from the focusing cupelectrode to either the cathode filament 22 or the anode 12, whichresults in damage to these elements. Thus, the cathode filament 22includes end leads 36 which may be insulated from the focusing cupelectrode 26 to enable a negative bias voltage of about 4 kilovolts tobe applied between such filament and such focusing electrode so that thex-ray tube is quiescently biased nonconducting.

The x-ray tube of FIGS. 1 and 2 may be connected in a capacitivedischarge x-ray circuit like that shown in FIG. 3, or such tube may beconnected to coaxial cables or to a transformer of high secondarywinding capacitance, so that a high capacitance on the order of onemicrofarad is connected across such tube. This capacitance acts as avoltage storage element which tends to cause an undesirable fieldemission or cold cathode" type of an electron discharge from thefocusing cup electrode 26 to the anode 12 or to the filament cathode 22.Thus, the anode 12 is connected through a first capacitance 38 toground, while the cathode 22 is connected through a second capacitance40 to ground. Capacitances 38 and 40 are charged slowly to about +60kilovolts and -60 kilovolts, respectively, through charging resistances42 and 44, respectively, and apply a total of 120 kilovolts D.C. acrossthe anode and cathode of the x-ray tube while the focusing cup isquiescently biased at -64 kilovolts DC. or 4 kilovolts relative to thecathode to cut off such tube. These capacitances are discharged rapidlythrough the x-ray tube to produce an x-ray pulse when an exposure pulse46 of about +4 kilovolts, relative to the quiescent voltage of thefocusing cup, is applied by an exposure pulser circuit 48 to thefocusing cup 26 to remove the quiescent reverse bias of 4 kilovoltsapplied to such focusing cup. Thus, the x-ray tube is renderedconducting for a period of time determined by the duration of theexposure pulse 46.

The storage capacitors 38 and 40 are connected through a rectifierbridge formed by four diodes 50, 52 54 and 56, across the secondarywindings 58 and 60 of a high voltage transformer 62. The transformer hasits primary windings 64 connected across the usual source of A. C. linevoltage 66. The transformer may also be provided with a low voltagefilament heater secondary winding 68 connected across the end terminalsof the cathode filament 22, as well as the filaments of the rectifierdiodes. It should be noted that a spark gap type of discharge switch maybe connected in series between the x-ray tube anode 12 and the capacitor38 when the focusing cup is connected to the cathode potential so thatexposure pulser circuit 48 is not employed to switch the x-ray tube intoa conducting condition. Such a spark gap discharge switch can betriggered or can be of the self-fire type which automatically breaksdown after the voltage on the capacitor reaches the desired value.

The following is a description of three different methods for applyingthe non-emissive coating 34 to the focusing cup electrode. In thepreferred embodiment of the invention, a non-emissive coating 34 ofplatinum or gold is deposited upon a focusing cup electrode base member32 of steel by sputtering. Any suitable sputtering method can beemployed, such as the triode sputter-ing method described in the articleLow-energy Sputtering by J. W. Nickerson and R. Moseson, inResearch/Development, March 1965, pages 52 to 56. Before sputtering, thefocusing cup is machined to the desired shape, cleaned by electrolyticpolishing and then dipped in a solution of trichloroethylene and placedin an ultrasonic cleaner containing liquid dichlorodifluoromethane, soldunder the trademark Freon. Next, the cleaned focusing cup members areinserted into the sputtering apparatus which is evacuated to less than 3X 10 torr, and are sputter etched for about ten minutes at 300 watts ofradio frequency power in an inert gas atmosphere of argon back filled to7 microns pres sure. In sputter etching the ions of inert gas are causedto bombard the surface of the focusing cup directly to remove any oxideor other foreign material and to thoroughly clean the surface for betteradherence to the platinum. Next, the etched focusing cup members areindexed over a target of platinum or gold and spaced about one-half inchtherefrom without removing them from the vacuum chamber. The target isthen sputter deposited onto the surface of the focusing cup for about 5minutes at about 1 kilowatt of radio frequency power in an argonatmosphere of 6.5 to 7.0 microns pressure to form the non-emissivecoating 34.

In triode sputtering, electrons are emitted from a cathode andtransmitted through argon or other inert gas to a separate anode,thereby ionizing the inert gas. The positive ions of inert gas areattracted to a target of the material to be sputter deposited which isat a more positive potential of about +50 volts so that the ions strikethe target with sufficient energy to cause platinum or gold atoms to besputtered from the target upon impact of such ions. These sputteredatoms of platinum or gold are transmitted in straight line paths to thefocusing electrode substrate member to form the sputtered coating 34. Itshould be noted that this sputter deposition technique has the advantagethat the platinum is not heated above its melting point in order tocause the coating 34 to adhere to the base material 32 which would causemelting of the base material due to the high melting point of platinum.However, once the sputtered platinum layer 34 is deposited, it may, ifdesired, be heated in a vacuum at about 1,000" Centrigrade for aboutseconds to enhance the surface condition of the layer and improve itsfusion with the base material 32. The filament cathode 22 is thenassembled in the coated focusing cup electrode 26, and this assembly ismounted within the x-ray tube for further processing includingevacuation and degassing at any suitable temperature.

Another method of depositing the non-emissive coating 34 is by ionplating, such as by the methods described in the article Film DepositionUsing Accelerated Ions by D. M. Mattox in Electrochemical Technology,Sept. Oct., 1964, pages 295 to 298, and in the article Gas-Scatteringand Ion-Plating Deposition Methods by Curt D. Kennedy et al. inResearch- /Development, November, 1971, pages 40 to 44. In ion plating,the platinum, gold or other high work function material to be depositedis first vaporized by heating and the metal vapor is ionized. Heatingand ionization may both be accomplished by electron beam bombardment ofabout 5 kilovolts of a target of the material to be deposited. The metalvapor ions are then accelerated through a high potential gradient ofapproximately 5 kilovolts to bombard the focusing cup electrode memberbeing coated. The positive ions of platinum or gold are imbedded intothe surface of the focusing electrode base member 32 to form thenon-emissive layer 34. In order to increase the number of metal vaporions generated, the secondary electrons emitted by the target may beutilized by placing the target in a magnetic field so that the secondaryelectrons travel in spiral paths through the metal vapor. In addition, aplasma or glow discharge can be produced between the substrate and theion source by supplying a small amount of inert gas and increasing thevoltage gradient. The ions of inert gas, as well as the ions of metalvapor, plasmaetch the surface of the substrate so that the nonemissivecoating 34 penetrates and adheres better to the base material 32 of thefocusing cup substrate. After ion plating, the coated substrate can, ifdesired, also be heated in a vacuum at about 1,000 Centrigrade for about30 seconds to enhance the surface condition of the layer and improve itsfusion with the base material as with sputtering. In addition, when goldis the sputter or ion plated material, the surface of the gold seems tobe made smoother and more continuous, apparently due to surface melting.

A third method of applying gold as the non-emissive coating 34 involveselectroplating and subsequent fusion in a vacuum. Prior toelectroplating the focusing cup electrode member 32 is machined,cleaned, and electrolytically polished. Then a thin intermediate nickellayer about 0.0001 inch thick may be flashed onto the steel to providebetter adherence of the gold non-emissive layer 34. However, this isoptional. Next the focusing electrode member 32 is electroplated withgold to a thickness on the order of 0.001 inch. After electroplating,the gold layer 34 is then fused to the focusing electrode member 32 byheating it above its melting point to approximately 1,070 Centrigradefor about 30 seconds in a vacuum by radio frequency heating or othersuitable heating techniques. This vacuum fusion also smooths the surfaceof the gold coating to provide a smooth continuous layer which furtherreduces the possibility of the field emission of electrons. After this,the filament cathode 22 is assembled in the coated focusing cupelectrode and such assembly is mounted in the x-ray tube, which is thenevacuated and baked for outgassing purposes.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the abovedescribed preferred embodiments of theinvention without departing from the spirit of the invention. Forexample, other high work function. materials than platinum and gold canbe employed as the non-emissive coating 34. Therefore, the scope of thepresent invention should only be determined by the following claims.

We claim:

1. An x-ray tube in which the improvement comprises:

an anode;

a focusing electrode;

a thermionic cathode mounted adjacent said focusing electrode so thatsaid focusing electrode focuses the electrons emitted by said cathodeonto said anode to cause x-rays to be emitted from said anode;

an evacuated envelope containing said cathode,

anode and focusing electrode; and

a layer of non-electron emissive material provided on at least a portionof the focusing electrode and having a higher work function than theunderlying base material of said focusing electrode to prevent theemission of electrons from said focusing electrode.

2. A tube in accordance with claim 1 in which the non-emissive materialis taken from the group consisting of gold and platinum.

3. A tube in accordance with claim 2 in which the cathode is a coiledfilament.

4. A tube in accordance with claim 3 in which the filament containsthoriated tungsten.

5. A tube in accordance with claim 1 in which the layer of non-emissivematerial is a sputtered layer.

6. A tube in accordance with claim 5 in which the sputtered layercontains platinum.

7. A tube in accordance with claim 1 in which the forming electrodeincludes a focusing cup within which a filament cathode is mounted, andthe layer of nonemissive material is provided on the surface of saidfocusing cup.

* alt

2. A tube in accordance with claim 1 in which the non-emissive materialis taken from the group consisting of gold and platinum.
 3. A tube inaccordance with claim 2 in which the cathode is a coiled filament.
 4. Atube in accordance with claim 3 in which the filament contains thoriatedtungsten.
 5. A tube in accordance with claim 1 in which the layer ofnon-emissive material is a sputtered layer.
 6. A tube in accordance withclaim 5 in which the sputtered layer contains platinum.
 7. A tube inaccordance with claim 1 in which the forming electrode includes afocusing cup within which a filament cathode is mounted, and the layerof non-emissive material is provided on the surface of said focusingcup.